Process for the conversion of NG to petroleum liquids are generally referred to as “Gas To Liquids” (GTL) processes. The overall conversion process involves a number of individual processing steps that are specific to the technology employed. The specific steps also depend on the feed stock used and the intermediate reaction products or processing pathway selected.
The processing steps generally follow the following sequence:    a. Feed stock supply processes—For conventional GTL plants, NG is directly extracted from a producing natural gas field via a number of production wells. The natural gas could, however, be supplied from compressed natural gas (CNG) sources or liquefied natural gas (LNG) storage tanks that involve re-vaporisation.    b. Feed stock conditioning processes—When the feed is directly from the well head, the NG has to be water and hydrocarbon dew point controlled, as well as treated for any acid gases present. With CNG and LNG feed stocks, the NG is already conditioned prior to compression and therefore further conditioning is not required.    c. Reforming processes—A large number of reforming process routes are available. Steam reforming, partial oxidation reforming and auto-thermal reforming are all variants of the same basic processing sequence to convert methane, CH4 (NG is typically 85% to 99% methane), to synthesis gas consisting of carbon monoxide, CO, and hydrogen H2. Synthesis gas could have a different carbon to hydrogen ratio depending on the conversion process used. The carbon to hydrogen ratio is important in terms of further downstream hydrocarbon chain growth reactions.    d. Ethylene cracking process—With the conversion of NG to liquids via the acetylene/ethylene route, a reforming process is not used to form carbon chain growth precursors or reactants. In this process methane, CH4, is cracked in the absence of oxygen under high temperature to acetylene, C2H2, which is hydro-treated with hydrogen to ethylene, C2H4. Ethylene is a multifunctional molecule containing the reactivity to facilitate carbon number chain growth under controlled conditions.    e. Synthesis gas conditioning—The synthesis gas is conditioned with any recycled gas to be a pure synthesis gas by removal of excess carbon dioxide, CO2 and nitrogen, N2, where necessary.    f. Hydrogen purification processes—Recycled gases containing a bulk composition of predominantly hydrogen, H2, have to be cleaned with a hydrogen concentration of at least 90 vol % before it can be blended in with synthesis gas as reactor feed gas. Hydrogen units are typically membrane or pressure swing absorption units, depending on the technology employed.    g. Air separation processes—This step depends on the reforming process used and whether pure oxygen is required in the reforming technology. Air separation processes applicable to GTL processing is therefore in general oxygen production plants used for reforming reagents. This is usually a proprietary pressure swing absorption, membrane or cryogenic separation process.    h. Main reaction processes—The heart of the GTL process is the conversion of conditioned reactor feed gas (C1 (CO) or C2) to longer chain length hydrocarbon products mostly in the C4 to C20 carbon number range. The most common processing route is via the catalysed Fischer-Tropsch process where the carbon number split is a function of the process temperature and pressure used, as well as the type of reactor and the catalyst (Iron versus Cobalt). With the ethylene reaction route, a basic hydrocarbon chain growth catalyst or acid zeolite is used, which is similar in function to the Fischer-Tropsch catalyst. The reaction mechanism is, however, different as oxygen is not present in the process molecules, i.e. the oxygen in the feed gas from the Reformer (as Carbon Monoxide, CO) is only applicable to Fischer-Tropsch reactions. With ethylene processing, only hydrocarbons are present and water does not form as a reaction product. Consequently this has some treatment plant benefits. Present state-of-the-art reactor designs have the catalyst in a fixed bed, moving bed, fluidised bed or slurry phase configuration.    i. Reaction products from the main reactor contain a mixture of carbon based compounds that have to be refined to commercial product specifications. This includes gasoline and diesel based products, as well as liquefied petroleum gas (LPG), kerosenes, aviation fuels, light end olefins and heavier waxes and cracker type feed products. These products are separated in a refinery using of a number of fractionation columns to separate predominantly on boiling point difference and distillation cut points.    j. Steam and electricity generation processes—Although all light end waste gas could be flared, it is typically used as an energy source to drive a boiler system or an electrical power generation unit.    k. Utility processes—Electricity or Steam co-generated by the use of waste gas is produced as a plant or complex wide utility for energy optimization. Light end waste gas could also be supplied as utility fuel gas for use in a number of burner units around the complex. Nitrogen separated from the Air separation process is supplied as a utility for safety inerting and blanketing purposes around the complex.